Digitizer tablets incorporating radiating magnetic fields have generally used two orthogonal arrays of conductors to form a grid matrix below the plane of the tablet, each array having the conductors disposed at precise regular intervals. Each array is energized by a periodic signal to produce a radiated field which is then received by a sensor within a movable cursor. The sensor position is determined by the comparison of the periodic signal energizing each array and the signal produced by the sensor. Since the magnetic field detected by the sensor is substantially different at the central area of each array, as compared to the magnetic field detected at the peripheral areas of the respective array, many digitizer tablets have restricted the area over which the sensor position may be accurately determined to a relatively small portion of the total grid matrix.
In digitizer tablets, each array conductor is energized by a periodic signal to produce a moving plane wave. The plane wave is produced by energizing an array first conductor for a specified period of time. When the first conductor is de-energized, the next adjacent array conductor is simultaneously energized for the same specified period of time; thereafter, when that conductor is de-energized, the subsequent conductor is simulatneously energized, and so forth. Other implementations have energized two or more adjacent conductors simultaneously. Regardless of the number of conductors simultaneously energized, the result of producing a plane wave by de-energizing one conductor while simultaneously energizing the other conductor results in the partial cancellation of the magnetic field by the collapsing field of the preceding de-energized conductor. In tablets having this partial cancellation of the magnetic field, extra circuit complexity is necessary to use the associated sensor pickup. Furthermore, inaccuracy in sensor position determination, especially near the tablet edge, is incurred. Other approaches used in digitizer tablets include complex filters used to condition the sensor signal according to the fixed magnetic field pattern resulting from the regular interval spacing of the array conductors, energizing the array conductor with high frequency alternating current signals, frequency modulating the high frequency current signals and redesigning the sensor. The result of these techniques is a physically large tablet (sometimes having several separate assemblies) having unnecessary complexity and critical elements.